1. Field of the Invention
The present invention relates generally to the art of bag making machinery, particularly to those rotary bag making machines which employ a rotary seal drum having one or more seal bars mounted thereon for sealing layers of plastic together to form bags. Still more specifically, the present invention relates to a rotary bag making machine which is adapted to form drawstring bags. Drawstring or draw tape may be used and the term "drawstring" is used to describe either in this specification.
2. Description of the Prior Art
Plastic bag machines have been known for a number of years, and the popularity of plastic bags for use as can liners, trash containers and for other use is well known. As of the time of the present invention, two main categories of plastic bag making equipment were in commercial use. The first type of bag making machine can be referred to as a shuttle type machine, in which the work piece (bag film material) is conveyed along a work surface to a location where it is to be sealed. When a seal is desired, a heated sealing bar is lowered to sandwich the plastic film between the bar and an anvil, a process which necessarily involves stopping the film, even if only very briefly. Dancer rolls and other complex devices accommodate the stop and start or jerky motion and precision work is quite difficult. Because of the sealing technique involved, the machines run at relatively slow speeds, e.g. 180 cycles per minute or less. With the average side sealed bag being 18" (between seals) the film speed of shuttle machine would be 270 feet per minute or less. These types of machines are known for making side or bottom sealed plastic bags.
The second type of bag machine known to the art is the rotary machine exemplified by the machines disclosed in U.S. Pat. No. 4,567,984 issued on Feb. 4, 1986 to Peter J. Gietman and entitled "Plastic Bag Package" and in its divisional U.S. Pat. No. 4,642,084 entitled "Plastic Bag Making Machine" and issued Feb. 10, 1987. Both patents are assigned to the assignee of the present invention. These devices include a rotary drum constructed of a plurality of slats and also includes a gear mechanism adapted for infinite variation of the drum diameter between a first smallest diameter and a second larger diameter. A sealing blanket is provided for the drum and is automatically adjusted for different drum diameters to provide proper tension against the film as it passes around the expandable drum. The patent discloses a mechanism for adjusting the diameter of the drum to facilitate the manufacture of a large number of different sized bottom and side seal bags of for insuring the correct drum diameter for a particular sized bag.
The seal bar(s) of the Gietman '984 and '084 machine comprise elongate metal sealing bars which are heated to the desired temperature and which may be spaced at various locations about the drum. For example, if two heating bars are employed, they are spaced 180.degree. from each other on opposite sides of the drum, and two bags may be made for each drum revolution. Depending upon whether certain seal bars are activated, and how many are employed, a huge variety of bag sizes can be made on the patented machine.
The assignee of the Gietman patents also owns a pending U.S. patent application, Ser. No. 07/194,654, filed on May 16, 1988, for "Bag Making Apparatus With Automatic Compensation System". In this application, a machine is disclosed which in many respects is similar to the aforementioned Gietman device, except that means are provided for adjusting drum diameter while the machine is operating. The machine can accordingly be used to make a variety of bags, including those of varying size and bags with other features such as printing, hand holes, other die cuts, etc.
In a preferred embodiment of the latter type of machine a once longitudinally folded strip of plastic film is fed to the sealing drum-blanket portion of the machine and seals are made transversely across the folded strip to form side seal bags. Marks provided on the film are photoelectrically sensed by the machine and a comparator causes adjustment of the drum diameter to ensure that the seals are being provided at the exact location from the print registration.
It is also known in the art that drawstring type bags may be made, and up to the present time have been made principally on the shuttle type machines described previously. A brochure describing such a system is entitled "Drawstring Bags from PE film with ribbon of HDPE" published by Windmoller & Holscher GmbH of Bielfield, West Germany. In this machine, a drawstring insertion system, such as the one patented and manufactured by AMI, Inc. of Doraville, Georgia is used to insert a tape made from high density polyethylene in an inwardly folded hem of a once longitudinally folded strip of polyethylene film. The system is described in U.S. Pat. No. 4,664,649 issued May 12, 1987 to Johnson, et al for "Method And Apparatus For Producing Drawstring Bags".
It will be appreciated by one familiar with such equipment that the sealing mechanism will encounter a six layer situation which can be described as follows: (1) lower outer bag layer (2) drawstring tape, (3) lower hem, (4) upper hem, (5) upper drawstring tape and (6) upper outer bag layer. With shuttle type bag machines it is necessary to seal (and in some cases burn through) the six layer sandwich so that the bag is formed and the tape ends are heat welded to one another. The sealing bar must heat the very thin bag material and the typically thicker and higher density tape material in such a way that the final product meets established standards, and the process is necessarily slower than desirable. While it is known that rotary bag machines can be run up to several hundred feet per minute faster than shuttle type machines, the increased speed of rotary machines would make it difficult to achieve the desirable welding of the six layer sandwich if conventional rotary equipment were to be used for drawstring bags. A rotary bag machine which is modified for rapidly producing exceptional quality drawstring bags would represent a significant advance in the art.